This invention relates to materials and methods for the manufacture of circuit materials and multilayer circuits.
Printed circuit boards must meet increasingly stringent performance and environmental requirements. For example, advanced printed circuit boards incorporate more layers and operate at increasingly higher frequencies. Thinner layers are desirable, but potentially increase dielectric or circuit related losses. To meet this need, substrates that have a dielectric constant (Dk) less than 4 can be used, as such substrates can accommodate higher signal propagation speeds, while minimizing insertion loss.
For environmental reasons, manufacturers seek to reduce heavy metals in circuit assemblies. Lead-free solders meet this need, but require higher process temperatures that are harmful to multi-layer circuit materials having a high coefficient of thermal expansion (CTE) or low thermal stability. Such materials can delaminate, chemically degrade, or simply melt. Furthermore, again for environmental reasons, manufacturers of high frequency electronic devices also seek to essentially eliminate the use of chlorinated and brominated flame retardants to achieve the UL-94 V0 flammability rating.
The requirements for low Dk, low loss, high temperature resistance, and inherent flame retardancy create a need for new types of circuit substrates. One class of materials that meets this need is the poly(aryl ether ketone) polymers (PAEKs). PAEKs include poly(ether ether ketone) (PEEK), poly(ether ketone), (PEK), poly(ether ether ketone ketone) (PEEKK), poly(ether ketone ketone) (PEKK), and similar materials. This class of materials exhibits high temperature resistance (with melting points of 307° C. to 381° C.), and reasonably low dielectric constant and loss values.
In addition to the above advantages, PAEK polymers are easily extruded into thin films, and have a low coefficient of thermal expansion (CTE). A good in-plane CTE match to copper foil laminate cladding can be achieved by adding relatively small amounts of mineral or ceramic filler to the PAEK polymers. A good in-plane CTE match is important to achieve dimensional stability in the circuit when the copper foil is selectively etched to form a circuit layer. PAEK polymers therefore exhibit many of the advantages of liquid crystalline polymer laminates such as Rogers Ultralam® 3000, with the added advantage of a higher melting point.
While PAEK is an excellent candidate as a dielectric material for the production of copper clad high frequency circuit laminates, its use requires the concomitant development of new bonding materials that also exhibit low Dk, low loss, high temperature resistance, and inherent flame retardance. Such bonding materials (bond plies) are used to bond two more circuit laminates to provide a multilayer circuit assembly.
A number of thermoplastic, low dielectric loss bonding materials are commercially available and are used to laminate high frequency circuit assemblies. However, these materials can have low melting points that limit the maximum use temperature or processing temperature of circuit assemblies. CuClad® 6250 bond ply, available from Arlon Corporation, restricts use temperatures in multi-layer circuits to a maximum of 75° C. CuClad® 6700 bond ply, also available from Arlon, has a melting point of 184° C. Circuit assemblies built with this material would not survive the 220° C. or greater temperatures required for lead-free solder assembly.
Higher melting point thermoplastic bond plies, including DuPont Teflon® FEP (melting point=250° C.), Teflon® PFA (melting point=308° C.), and Rogers Ultralam® 3908 melting point=280° C.), have also been used in the manufacture of low loss, high frequency circuit boards. Although high melting substrates simplify the fabrication of multi-layer circuits, other factors such as the substrate modulus can be limiting. For example, a typical substrate material, Rogers Ultralam® 3850 (melting point=315° C.), can be used with a bond ply such as Ultralam® 3908. While there is a 35° C. temperature window between the melting points of this substrate and bond ply, the low modulus of the substrate at high temperature leads to distortion of the substrate during bonding. This distortion causes poor registration between the layers of the circuit assembly. On the other hand, if a substrate such as DuPont Teflon PFA film is used with a bond ply such as Ultralam® 3908, the 7° C. temperature window between the melting point of the substrate and that of the bond ply is too small to allow multi-layer circuits to be fabricated in an industrial process.
Thermosetting bond plies are also widely used in the multi-layer circuit industry. In recent years, the most commonly used material is “FR4,” a family of flame retardant, epoxy fiberglass composite substrate materials and bonding plies. The glass transition temperature (Tg), decomposition temperature, and high frequency electrical properties depend on the precise formulation of the epoxy resin. Typically, the least expensive grades of FR4 that are used in lower frequency consumer products exhibit a Tg of about 125° C. and a dissipation factor (DF) at 10 GHz of greater than 0.02. Multi-layer circuits made with PAEK substrates and FR4 epoxy (Tg=125° C.) cannot survive the high temperature required for lead-free soldering. Such multi-layers also exhibit DF values that are too high for use in many high frequency devices.
The Isola Group manufactures a high performance FR4 formulation known as FR408, which exhibits a Tg of 180° C. and a DF of 0.013 at 10 GHz. Even with these improved properties of the thermosetting bonding ply, a circuit assembly made with this material and PAEK would not exhibit high performance. The lead-free solder temperature resistance would not be high enough for robust manufacturing nor would the DF of the multi-layer assembly be low enough for many high frequency applications.
Other low dielectric polymers are presently used as circuit materials, including phenol-formaldehyde resins, epoxy resins, and isoprene- and butadiene-based resins. While suitable for their intended purposes, there remains a continuing perceived need in the art for circuit materials with improved combinations of properties, in particular materials that have a combination of low dielectric constant and low loss, high temperature solder resistance, and flame retardance in the absence of brominated and/or chlorinated flame retardants.